1. Field of the Invention
The present invention relates generally to imaging devices, and more particularly to a media feed roll assembly for an imaging device.
2. Description of the Related Art
Media feed roll assemblies move sheets of media along a media feed path extending between a media input location, such as a media input tray or multipurpose tray, to a media output location in an imaging device. In such assemblies, shafts are mounted parallel to each other within a frame of the assembly and mount one or more opposed rolls forming a feed nip therebetween for feeding a media sheet. During media feeding, unwanted skew of a media sheet may occur due to varying nip pressure or shaft misalignment or skew.
A prior art media feed roll assembly 10-1 is shown in FIG. 1A. Assembly 10-1 includes opposed driven roll 12 and idler roll 14 forming a nip 16 and having respective rotational centers aligned along a common centerline 20. A leaf spring 30-1 is mounted between frame supports 40 and provides a biasing force to idler roll 14 along common centerline 20. The amount of biasing force applied to idler roll 14 is proportional to the cube of the thickness T of leaf spring 30-1. The biasing force applied to idler roll 14 is exerted on driven roll 12 or on the media sheet while it passes through nip 16. The idler roll 14 moves up and down as indicated by arrow 22, i.e. into and out of contact with, the driven feed roll 12 whenever the media sheet passes through the nip 16.
Another prior art assembly 10-2 is shown in FIG. 1B again having driven roll 12 and idler roll 14 forming nip 16, both being positioned along common centerline 20. Here, leaf spring 30-2 is mounted in a cantilever arrangement having one end affixed to support 40 while the free end thereof contacts idler roll 14 to bias it toward driven roll 12 generally along common centerline 20. In the prior art designs of FIGS. 1A-1B, idler roll 14 is prone to misalignment from common centerline 20 because each component (e.g., driven roll 12) along the length of its drive shaft has part tolerances that affect idler roll 14 as it is biased toward driven roll 12. Alignment control of the driven roll 12 and idler roll 14 is further affected as additional bushings are used to mount the shaft(s). Accounting for such tolerance stack-ups and having controllable and predictable force application are desired for proper alignment of the two opposed rolls.
With increasing need to reduce the size of imaging devices, the space for the media path and the media feed roll assemblies is preferably reduced. In turn, space to place biasing features, such as the leaf springs and torsion springs, is reduced. Likewise, any increased functionality provided in an imaging device may use the space that once provided biasing support features. Also, while leaf springs are used to help reduce the space needed for loading features their use introduces variability in the characteristics of the spring due to variations in the thickness T of such leaf springs from part to part. The biasing force for a given leaf spring is proportional to the cube of its thickness T. As would be appreciated, a small variation in the thickness creates a much larger variation in the spring force.
In order to address part to part variations when loading the idler shaft, a more recent assembly 10-3 shown in FIG. 1C uses an extension spring 32 wrapped partially around a bushing 15 on idler shaft 17 of idler roll 14. The ends of the extension spring 32 are fixed to mounts 40. However, as idler roll 14 moves during media feeding and extension spring 32 stretches around bushing 15, friction between spring coils 33 and bushing 15 prevents some of the spring coils 33 from moving, resulting in force variations and unbalanced pressures along the length of nip 16. Also, over time, the friction between extension spring 32 and bushing 15 changes, resulting in further variation of the nip pressure. Another prior art design uses a spring extended between the two shafts that is hooked onto a bushing supporting the shafts. However, the narrow space therebetween forces the spring to have a small number of coils and heavy gage wire. This produces a large spring rate resulting in an unstable and unpredictable nip loading.
A further prior art assembly 10-4 shown in FIG. 1D uses a torsion spring 34 to bias idler roll 14. The body 35 of torsion spring 34 is mounted to a cantilever post 42 positioned to the side of idler roll 14. A first leg 36 of torsion spring 34 engages idler roll 14 and a second leg 37 engages the support 40. Two drawbacks with this arrangement are the amount of space needed to mount torsion spring 34 and the strength required for post 42 on an imaging device frame to withstand the spring forces, such as F′ and F″, involved. Typically, support 40 and the frame on which post 42 is mounted are made of plastic which requires that both be reinforced to withstand the forces involved. In still another prior art design, a class 2 lever is positioned in the same manner as torsion spring 34 but extends beyond idler roll 14 to provide a fulcrum on one end at support 40 and a resistance in the middle for biasing the idler roll 14. However, a mechanism to produce effort on the other end of the lever uses additional parts and requires more space, thereby requiring a higher profile than the previous spring arrangements shown.
It would be advantageous to employ a biasing spring in a media feed roll assembly where part to part variation is minimized. It would be further advantageous to have an alignment and biasing assembly that provides a low profile and which minimizes space requirements within the imaging device. It would be still further advantageous to have a spring mounting arrangement that does not require an additional mounting post.